The Fire in the upper floors didn't melt the structural steel. It simply softened it up and gravity did the rest.
Pools at the bottom are a different thing all together. Drop half a million tons from 1300 feet and you're going to create a massive amount of heat. Enough to melt just about everything.
The interesting thing is that physics buffs can calculate the potential energy released from the building collapse. All the calculations of which I am aware show that the numbers don't add up, particularly when one takes into account the energy required to turn many floors of concrete into a pyroclastic cloud of dust. Then there are the peculiar seismographic readings, showing the biggest spikes at the beginning of the collapses, not the ends. . .